1. Field of the Invention
The present invention relates to a Voltage Controlled Oscillator (VCO). More particularly, the present invention relates to a VCO for determining an output frequency by controlling a capacitor value in a cardinal manner.
2. Description of the Related Art
A VCO is a device for outputting a frequency that depends on an externally applied voltage. The VCO is primarily used for an analog sound synthesis apparatus, a mobile communication terminal, and the like. The VCO used for a sound synthesis apparatus generates a sine wave, a sawtooth wave, a pulse wave, a square wave, a triangle wave, and the like to generate a basic sound. The VCO is used for a Phase Locked Loop (PLL) module in a mobile communication terminal to serve as a local oscillator for converting a frequency into a Radio Frequency (RF) or an Intermediate Frequency (IF).
The VCO includes an inductor (L), a capacitor (C), and a negative-gm unit. In an LC-VCO, for realizing negative-gm, an N-type Metal-Oxide-Semiconductor (NMOS) transistor and a P-type Metal-Oxide-Semiconductor (PMOS) transistor are connected in a cross-coupled pair. Because the LC-VCO can obtain a negative-gm that is twice greater than a structure that uses only an NMOS, power consumption may be reduced.
In addition, in an LC-VCO circuit, a resonance frequency has a value inversely proportional to √{square root over (LC)}. According to the related art, an inductor is fixed as a constant, and an output frequency is controlled by changing a capacitor value. A frequency range is set using a capacitor band having a binary structure and a switch device, and a fine frequency control is performed using a varactor diode, and the like.
According to the related art, a frequency of a VCO in a wireless terminal corresponds to a log scale. The frequency of the VCO is proportional to 1/√{square root over (LC0)} depending on a digital code (k) and changes non-linearly in the log scale.
FIG. 1 is a graph illustrating a frequency of a voltage controlled oscillator that depends on a digital code according to the related art.
Referring to FIG. 1, an x axis represents a digital code value determining a capacitor value, and a y axis represents an output frequency of a VCO that depends on a capacitor value which varies according to a digital code value.
FIG. 1 illustrates an example in which when the digital code changes from 1 to 256, a frequency of a VCO changes by 0.1˜3 GHz. As illustrated in FIG. 1, as a digital code increases, the frequency of the VOC decreases non-linearly in the log scale.
Assuming that a digital control code is k and a capacitor minimum unit is C0, capacitance of a capacitor bank becomes kC0, and accordingly the frequency of the VCO has a value proportional to 1/√{square root over (kC0)}. Because capacitance change of a varactor diode is less than 100 fF, a minimum unit C0 should be smaller than 100 fF, and a capacitor band of about 16 bits is required to have a range controlling all of 0.7˜3 GHz which are commercial wireless terminal frequencies. However, when a capacitor band becomes 16 bits, a phase noise characteristic is deteriorated very much by parasitic capacitance by a switch device. In addition, accuracy is lowered by a capacitor band change of 216. Therefore, a circuit is designed using two or more VCOs that depend on a frequency range so that the controller is controlled depending on a 7˜8-bit control code.
However, because two or more VCOs are used, a circuit area increases at least twofold. Consequently, manufacturing costs increase and power efficiency of the circuit is reduced.
Therefore, a need exists for an apparatus, system and method for providing a VCO circuit that effectively controls a frequency via a capacitor bank whose frequency range linearly corresponds to dB unit given as a log scale.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.